Low-speed flight phases are dangerous for aircraft, in particular during the landing and takeoff phases where the margin for maneuver in relation to the terrain is all the more critical as the wind currents are strong and the altitude is low. To preclude abnormal changes of speed and of attitude of the airplane, onboard systems for detecting airplane stall are already known. But, at low altitude, they do not offer the pilot the possibility of sufficiently early anticipation of collision with the ground. Moreover, these stall detection systems remain simply reactive (these detection systems are triggered even though the aircraft is already in a poor posture, in contradistinction to predictive systems which are triggered before the aircraft is in danger).
Furthermore, these systems are too dependent on incidence probes. This poses a problem notably in so-called crab approach phases, that is to say in a crosswind. If the aircraft exhibits a single incidence probe situated in a zone of turbulence caused by the fuselage of the aircraft, then the incidence probe risks delivering erroneous information. This is why most aircraft have two incidence probes situated on either side of the aircraft. The angle of incidence value used by the flight management systems is then an average of the angles of incidence provided by the two probes. This arrangement limits the errors but causes discrepancies between the real value of the angle of incidence and the averaged value.
Problems with probes are at the origin of accidents occurring in so-called CFIT (Controlled Flight Into Terrain) normal approach phases, in the course of which the pilots and the flight systems fully control the aircraft but have a false idea of its situation in the vertical and/or horizontal plane. Typically, if a probe delivers erroneous information, the automatic pilot may believe that the aircraft is on the ground. The automatic pilot then cuts the engines. The pilot does not then have time to react and the aircraft stalls.